Exploring the Mechanical Anisotropy and Ideal Strengths of Tetragonal B₄CO₄.

First-principles calculations were employed to study the mechanical properties for the recently proposed tetragonal B₄CO₄ (-B₄CO₄). The calculated structural parameters and elastic constants of -B₄CO₄ are in excellent agreement with the previous results, indicating the reliability of the present calculations. The directional dependences of the Young's modulus and shear modulus for -B₄CO₄ are deduced in detail, and the corresponding results suggest that the -B₄CO₄ possesses a high degree of anisotropy. Based on the strain-stress method, the ideal tensile and shear strengths along the principal crystal directions are calculated, and the obtained results indicate that the shear mode along (001)[100] slip system dominates the plastic deformation of -B₄CO₄, which can be ascribed to the breaking of the ionic B-O bonds. The weakest ideal shear strength of 27.5 GPa demonstrates that the -B₄CO₄ compound is not a superhard material, but is indeed a hard material. Based on the atomic explanation that the ternary B-C-O compounds cannot acquire high ideal strength, we propose two possible routes to design superhard B-C-O compounds.